1. Field of the Invention
The present invention relates to a circularly polarized wave generator used in transmitter-receivers for satellite broadcasting systems, etc., and particularly to a circularly polarized wave generator using a dielectric plate as a 90xc2x0 phase shifter.
2. Description of the Related Art
FIGS. 9A and 9B show such a conventional circularly polarized wave generator, FIG. 9A is its left side view, and FIG. 9B is its sectional view. This conventional circularly polarized wave generator is provided with a waveguide 10 in which one end is open and the other end is closed and a dielectric plate 11 is arranged inside the waveguide 10, and a pair of probes 12 and 13 are inserted inside the waveguide 10 through its external wall surfaces. The waveguide 10 is a rectangular waveguide having a square cross-section the inside of which is hollow, and the feature of such a rectangular waveguide is that, as compared to a circular waveguide having a circular cross-section, for example, the area of a Printed Circuit board (not illustrated) to which the probes 12 and 13 are connected can be reduced. The dielectric plate 11 functions as a 90xc2x0 phase shifter, and is composed of a dielectric material having a uniform thickness. The dielectric plate 11 is fixed at two corner portions located on a diagonal plane inside the waveguide 10, and both ends in the longitudinal direction of the dielectric plate 11 are cut to be V-shaped so that the input and output impedance matching is improved. The two probes 12 and 13 are orthogonal to each other and the dielectric plate 11 is tilted about 45 degrees with respect to the probes 12 and 13.
In the circularly polarized wave generator constructed in this way, a circularly polarized wave input into the waveguide 10 can be converted into a linearly polarized wave to output the linearly polarized wave, and, on the contrary, a linearly polarized wave input into the waveguide 10 can be converted into a circularly polarized wave to output the circularly polarized wave. That is, since a circularly polarized wave is a polarized wave in which a composite vector of two linearly polarized waves having the same amplitude and a 90-degree phase difference therebetween rotates, when the circularly polarized wave passes through the dielectric plate 11, the waves having a 90-degree phase difference from each other are made to have the same phase and are thus converted into a linearly polarized wave. In the example shown in FIGS. 9A and 9B, since a left-handed circularly polarized wave input to the waveguide 10 is converted into a vertically polarized wave and a right-handed circularly polarized wave is converted into a horizontally polarized wave, the vertically polarized wave and horizontally polarized wave are received by the probes 12 and 13, respectively, which are arranged to be orthogonal to the planes of polarization and the received signals can be frequency converted into intermediate-frequency signals by a converter circuit (not illustrated) to output the intermediate-frequency signals.
In the circularly polarized wave generator having the construction as described above, the electric field distribution inside the waveguide 10 having a square cross-section is as shown in FIG. 10. As clearly shown in the drawing, the electric field E1 (broken line) and the electric field E2 (solid line) are arc-shaped distributions of field strength centered on the corner portions of the waveguide 10, and it can be understood that the electric field E1 does not exist at both edge portions of the dielectric plate 11 which are fixed to the corner portions of the waveguide 10. This is because the electric fields E1 and E2 on each flat surface of the waveguide 10 are perpendicular to its surface in the nature of things, and, as a result, the polarized wave component propagating in the dielectric plate 11 decreases. For such a reason, it is required to sufficiently lengthen the dielectric plate 11 along the central axis of the waveguide 10 so that the waves having a 90xc2x0 phase difference from each other may be made to have the same phase by the dielectric plate 11, and this fact has been a major obstacle to miniaturization of circularly polarized wave generators. Moreover, such a problem becomes obvious when waveguides having a square cross-section are used, but also the problem can be seen in the same way when waveguides having a circular cross-section are used.
The present invention has been made by considering such facts about the conventional technology, and it is an object of the present invention to provide a circularly polarized wave generator in which the length of a dielectric plate as a 90xc2x0 phase shifter is reduced and which is appropriate for miniaturization.
In order to attain the above object, a circularly polarized wave generator of the present invention comprises a first waveguide having a square opening at one end thereof, a dielectric plate arranged inside the first waveguide so as to be substantially orthogonal to two parallel sides of the opening, and a second waveguide having a square cross-section coaxially connected to the other end of the first waveguide, and is characterized in that the dielectric plate is tilted substantially 45xc2x0 with respect to the inner wall surfaces of the second waveguide.
In the circularly polarized wave generator constructed in this way, since the dielectric plate arranged inside the first waveguide is tilted substantially 45xc2x0 toward the flat surfaces of the second waveguide and the dielectric plate is substantially orthogonal to the parallel two sides of the opening of the first waveguide, even if the length of the dielectric plate is shortened, a phase difference between orthogonal polarized waves becomes large and the circularly polarized wave generator can be made smaller. In this case, the opening of the first waveguide is preferably square, but also regular polygons such as a regular hexagon, a regular octagon, etc., in which opposing two sides are parallel to each other, can be used.
In order to attain the above object, a circularly polarized wave generator of the present invention comprises a first waveguide having a circular opening at one end thereof, a dielectric plate arranged inside the first waveguide, and a second waveguide having a square cross-section coaxially connected to the other end of the first waveguide, and is characterized in that the dielectric plate is tilted substantially 45xc2x0 with respect to the inner wall surfaces of the second waveguide.
Also in the thus constructed circularly polarized wave generator, since the dielectric plate arranged inside the first waveguide is tilted substantially 45xc2x0 toward the flat surfaces of the second waveguide, even if the length of the dielectric plate is shortened, a phase difference between orthogonal polarized waves becomes large and the circularly polarized wave generator can be made smaller.
In each of the constructions described above, the second waveguide is preferably inscribed in the opening of the first waveguide so that the corner portions of the neighboring inner wall surfaces of the second waveguide are on the boundary of the opening, and, when constructed in this way, a first waveguide and a second waveguide which are connected in the axial direction can be easily produced by rolling out and expanding part of a waveguide having a square cross-section.